MS gurus, how easy/hard is it to turn on IGMPv3?

So I need to rollout a quick PIM-SSM solution (as in, very quick). Designing and verifying a PIM-SM solution will simply take too long, but a PIM-SSM solution would be quick....HOWEVER...it seems XP/Win7 don't send IGMPv3 joins by default. A quick googling tells me it's a simple registry change (add a statement to the $reg/tcpip/parameters registry thing).

Can anyone confirm/deny? Would this be something easily done via SCCM (or one of those types of tools?)

Have you done it? Did it break all the things, shoot your dog, and key your car?

Well, the main goal here is to rid ourselves of RPs and the associated complexity of PIM-SM...so I'm unclear as to whether or not this an issue. (The main requirement being to use source-specific joins so as to avoid hijacking...a highly unlikely event in the first place, but one that was added to the reqs nonetheless)

I was unaware of the fallback behavior...it honestly never even occurred to me.